Aqueous cutting fluids are defined by international standard ISO 6743/7--(T60-504)--1986. They comprise emulsions or microemulsions made from concentrates which are mixtures of emulsifying agents and appropriate additives together with refined mineral oils, diluted in water.
In use, these aqueous cutting fluids are contaminated with metal particles that come from the machining of metal parts, by foreign oils leaking from the machine, and also by bacteria. In order to limit this contamination, coarse purifying devices exist for the purpose of removing physical contaminants (swarf, debris) and foreign oils.
Physical contaminants may be removed by: PA1 magnetic separators; PA1 dragging the dirty emulsion in the tank containing the emulsion; PA1 filtering under pressure or suction through paper filters or through diatom filters; PA1 hydrocyclones; or PA1 centrifugal purifiers. PA1 Foreign oils may be removed by: PA1 skimming the used emulsion tank; or PA1 centrifuging either the entire emulsion or else the surface layer thereof as obtained via a floating strainer.
None of these mechanical coarse purifying systems is completely effective and a portion of the above-mentioned pollutants remain in the cutting fluid.
When the contamination becomes too concentrated, aqueous cutting fluids no longer perform their functions properly: they are then replaced and sent to an officially agreed treatment center for disposal, and this takes place periodically. It is illegal to discard any oil, emulsified or otherwise into the drainage system since discarding such used fluids into the natural environment constitutes a danger by virtue of the chemical composition thereof and also because of the pollutants contained therein.
Tangential filtering methods are known for processing emulsions using various types of support associated with ultrafiltration membranes having a pore diameter lying in the range 40 .ANG. to 1000 .ANG..
For example, it is mentioned in the "Guide d'emploi des fluides de coupe" (Cutting fluid user guide) published by CETIM, volume 1, 1979, page 183, that the ultrafiltration module then separates an ultrafiltrate which is practically free from oil and a residue having a very high oil concentration which is unusable as a cutting fluid and which is to be destroyed.
It is also mentioned by B. Bartoli (C.E.A.) in an article "La separation des emulsions par ultrafiltration" (Separating emulsions by ultrafiltration), taken from a CETIM document entitled "Les fluide de Coupe aqueux" (Aqueous cutting fluids), Nov. 21-23, 1978, page 101, that an ultrafiltration method separates a polluted emulsion into an aqueous phase and an oily phase that is to be dstroyed.
In other words, if a tangential ultrafiltration method is applied to cutting fluids, the pollutants are indeed removed, however the oily phase is separated from the aqueous phase, in other words the emulsion is destroyed.
The object of the present invention is to provide a method of treatment by tangential filtration enabling pollutants to be removed without destroying the emulsion or the microemulsion of oil in water, in other words enabling said emulsion to be regenerated.